1. Field of the Invention
The present invention generally relates to the art of optical data communications, and more specifically to an automatic bias controller for an electro-optic modulator such as a Mach-Zehnder modulator.
2. Description of the Related Art
Transmission of data using optical carriers enables very high bandwidths and numbers of multiplexed channels with low signal loss and distortion. A coherent laser light beam is amplitude modulated with a data signal, and propagates to a remote receiver either directly through the atmosphere, or via a system of optical fibers and repeaters. The light beam may advantageously be modulated with electrical signals in the microwave frequency range using an electro-optic modulator such as a Mach-Zehnder modulator or optical coupler.
A Mach-Zehnder electro-optic modulator per se is known in the art, as presented in an article entitled "A new waveguide switch/modulator for integrated optics", by W. Martin, Applied Physics Letters, vol. 26, no. 10, pp. 562-564 (May 1975). An electro-optic modulator based on a Mach-Zehnder interferometer generally includes a monolithic substrate formed of an electro-optic material such as LiNbO.sub.3 or ZnSe. An optical waveguide is formed in the substrate having two arms or branches which extend generally in parallel with each other. The index of refraction of the material in the waveguide is higher than the index of refraction of the material of the substrate.
The branches have equal lengths. In the absence of an applied electrical bias voltage, an input optical or light beam produced by a laser or the like applied to the waveguide divides equally between the branches. The optical signals propagating through the branches recombine at the optical output of the waveguide in phase with each other, such that their amplitudes are additive and an optical output signal which is essentially similar to the optical input signal appears at the output of the waveguide.
Application of a predetermined electrical bias voltage differential to one branch of the waveguide relative to the other branch causes the indices of refraction of the material in the branches to vary differently due to the electro-optic effect, such that the effective optical lengths of the branches vary accordingly. At a bias voltage known in the art as V.pi., the effective optical lengths have varied to such an extent that the optical signals emerging from the branches are 180.degree. out of phase with each other. The amplitudes of the signals combine subtractively, cancelling each other out, such that a zero output is produced at the optical output.
For most optical communication applications, it is desirable to bias the modulator at a voltage V.pi./2, which produces linear operation. However, device instabilities and environmental effects, especially temperature variations, cause the operating point to drift, and constant manual readjustment has been required to maintain the proper linear operating point. The linear bias point must be maintained during link operation to achieve maximum dynamic range, since second order harmonic and intermodulation distortion increase rapidly with increasing bias voltage error.
An electro-optic modulator system including a provision for manual bias voltage adjustment is illustrated in FIG. 1, and generally designated as 10. A laser 12 feeds a coherent light beam through an optical fiber 14 into an optical input 16 of a Mach-Zehnder modulator 18, optical coupler, or other appropriate electro-optic modulator. The light beam propagates through a waveguide having two branches 20 and 22, which recombine at an optical output 24 of the modulator 18. An electrical data signal, preferably in the microwave frequency range, is applied to the branch 20 via an electrical input line 25 and modulation signal "T" input 26. The optical carrier signal constituted by the laser beam is modulated with the data signal, and fed through an optical fiber 28 to a remote receiver (not shown).
In order to control the modulator 18 to linearly modulate the optical carrier with the data signal, it is necessary to bias the modulator 18 at a linear operating point corresponding to the voltage V.pi./2. In the prior art arrangement illustrated in FIG. 1, a tap 30 is provided in the output optical fiber 28 which leads through an optical fiber pigtail 32 to a photodetector 34. A second optical fiber pigtail 36 leads from the substrate of the modulator 18, and is designed to feed a portion of the light scattered into the substrate to a second photodetector 38. The photodetectors 34 and 38 produce electrical signals which are complementary inverses of each other. The correct bias point is initially determined by a complex manual procedure whereby both photodetector outputs are measured at the correct operating point. Manual adjustment of the bias voltage by means of a potentiometer 40 or the like, of a manual bias voltage controller 42, is performed to maintain the ratio of the two detector outputs constant. Constant monitoring and manual adjustment of the bias voltage is required to correct for variations in the operating point of the modulator 18. It is not feasible to implement the prior art system 10 as a practical industrial product.